The latent heat of fusion of ice is about:

Latent heat is the heat energy absorbed or released by a substance during a change of state (phase change), such as melting, freezing, boiling or condensing, without any change in its temperature. The word "latent" means "hidden", because this heat does not show up as a rise in temperature on a thermometer. It is used to break (or form) the bonds between particles rather than to increase their kinetic energy.

Latent heat of fusion is the heat needed to change 1 gram (or 1 kilogram) of a solid into liquid at its melting point, without any change in temperature. For ice at 0°C, it is about 80 calories per gram, or about 3.34 × 10⁵ joules per kilogram (334 kJ/kg). This means that 80 calories of heat are needed to turn 1 gram of ice at 0°C into 1 gram of water at 0°C. When water freezes, the same amount of heat is released.

Latent heat of vaporisation is the heat needed to change 1 gram of a liquid into vapour at its boiling point. For water at 100°C, it is about 540 calories per gram, or about 2.26 × 10⁶ J/kg (2,260 kJ/kg).

This explains several everyday observations:

• Ice at 0°C cools drinks better than water at 0°C, because ice absorbs extra heat (latent heat) while melting.
• Burns from steam at 100°C are more severe than burns from boiling water at 100°C, because steam releases its large latent heat of vaporisation when it condenses on the skin.
• Sweating cools the body, because evaporation of sweat absorbs latent heat from the skin.
• The temperature stays constant during melting and boiling until the change of state is complete.
• The concept of latent heat was introduced by the Scottish scientist Joseph Black around 1761.

Why not the other options?

• ✗540 cal/g is the latent heat of vaporisation of water.
• ✗100 cal/g is incorrect.
• ✗1 cal/g per °C is the specific heat of water.